1. Field of the Invention
The invention relates to a device for concentrating radiant energy and somewhat more particularly to a device for collecting light and a method of producing such a device.
2. Prior Art
Devices for collecting light having, for example, a plate-shaped body (sometimes referred to as a "fluorescent" plate or body) functioning as a light trap having at least one light-exit window and comprised of a solid polymerized synthetic material having fluorescent particles therein are known in numerous embodiments and are useful, for example, for concentrating and collecting solar energy [P. B. Mauer and G. T. Turechek, Research Disclosure Vol. 129, paragraph 12930 (1975); German Offenlegungsschrift No. 26 20 115 (generally corresponding to U.S. Patent 4,110,123); or A. Goetzberger and W. Greubel, Applied Physics, Vol. 14, pages 123-139 (1977)], for optical indicia transmission (G. Baur et al, U.S. Ser. No. 932,569 filed Aug. 10, 1978), for image brightening of passive displays [German Offenlegungsschrift No. 25 54 226 (generally corresponding to U.S. Pat. No. 4,142,781) or W. Greubel and G. Baur, Elektronik Vol. 6, pages 55-56, (1977)], or for increasing the sensitivity of scintillators [G. Kell, Nuclear Instruments and Methods, Vol. 87, pages 111-123, (1970)].
In such devices, when light strikes a fluorescent plate, the light spectrum portion which is in the excitation spectrum of the fluorescent particles within the plate is absorbed by the fluorescent centers and the remaining portion of the light spectrum permeates the fluorescent plate without disturbance. The so-absorbed radiation, shifted toward longer wavelengths and spatially undirected, is re-emitted from the fluorescent centers. By far the greatest proportion of this fluorescent light is piped in the interior of the fluorescent plate via total reflections on the plate interfaces until it emerges at specific output areas with an increased intensity.
The efficiency achieved with presently available fluorescent plate still lags significantly behind theoretically possible values, primarily because the emission spectrum overlaps the absorption spectrum so that the fluorescent radiation in the plate has a finite absorption length. This "self-absorption" is particularly unsatisfactory with fluorescent bodies having a large collecting surface.
Workers in the field are aware that many organic fluorescent materials cause a shift of the emission spectrum toward lower frequencies, relative to the excited spectrum, when such materials are dissolved in a liquid having a strongly orientating polarization effect, i.e., a so-called red shift. Such red shift occurs when a fluorescing molecule has different dipole moments, .mu..sub.g in the basic or rest state and .mu..sub.a in the excited state, so that when .mu..sub.g is not equal to .mu..sub.a and when the polarization of the environment about such particle or molecule (which remains unchanged during the absorption process) can re-orientate during the existence of the excited state [see E. Lippert, Zeitschrift der Elektrochem. Ber. Bunsengesellschaft Phys. Chem., Vol. 61, pages 962-975, (1957)]. If .mu..sub.a is substantially greater than .mu..sub.g, then one can readily observe a red shift of the emission spectrum, given an essentially unchanging position of the absorption spectrum. A blue shift of the absorption occurs, given an essentially unchanged position of the absorption spectrum when .mu..sub.g is substantially larger than .mu..sub.a.
Fluorescent bodies are preferably comprised of a solid carrier material. Such solid carriers, particularly when they are synthetic organic materials, can be readily manufactured and processed with relatively low economic outlays, which is a very significant advantage, particularly in mass production.
That a desired spectrum band separation also depends on the dielectric constant (.epsilon.) of a solvent in solid body solution and consequently the dipole difference in the basic and excited state plays an important role is suggested by the earlier cited Goetzberger and Greubel article in Applied Physics, Vol. 14, (1977), (cf. Section 3.3 therein). However, knowledge of how the suggested interrelationships might allow one to attain solid fluorescent bodies from a synthetic base materials with a high orientation polarization in actual practice is still absent. As a general rule, highly transparent synthetic materials which are presently available have only relatively small .epsilon.-values, since up to now it was primarily a matter of achieving good electrical insulating properties.